code: def holdout_df():
      return pd.DataFrame(
          {
              "id": ["id4", "id4", "id5", "id6"],
              "x1": [13.0, 10.0, 13.0, 10.0],
              "x2": [1, 1, 0, 1],
              "x3": [13.0, 10.0, 13.0, 10.0],
              "x4": [1, 1, 0, 1],
              "x5": [13.2, 10.5, 13.7, 11.0],
              "x6": [1.4, 3.2, 0, 4.6],
              "w": [1, 2, 0, 0.5],
              "target": [1, 0, 0, 1],
          }
      )
  
file_path: fklearn/tests/tuning/test_samplers.py
imports: import pandas
  import pytest
  from tests import LOGS, PARALLEL_LOGS
  from toolz.curried import first
  from fklearn.metrics.pd_extractors import evaluator_extractor
  from fklearn.training.classification import logistic_classification_learner
  from fklearn.tuning.samplers import remove_by_feature_importance, remove_by_feature_shuffling, remove_features_subsets
  from fklearn.validation.evaluators import roc_auc_evaluator
label_desc: Use image node_id 6 for calling a global function with example usage: holdout_df() and returns: pd